For the last three decades there has been an increasing interest in and demand for metallic alloys that are resistant to chloride solutions as well as to a broad spectrum of other corrosive solutions. For example, power plants and chemical process equipment are now often cooled by seawater or brackish water. In addition, metallic alloys resistant to seawater are useful in many ship, submarine, dock, drilling and platform construction applications. It is, of course, desirable that these alloys bear the lowest cost consistent with meeting the requirements of each application.
A now widely accepted index of corrosion resistance of alloys to chloride solutions has been developed. This index has a numerical value derived by adding together chromium content plus 3.3 times molybdenum content plus 16 times nitrogen content, all contents being weight percentages of these elements in a given alloy. This relationship has proven to be a very useful indicator of the relative corrosion resistance of alloys composed primarily of iron, nickel, chromium, molybdenum and nitrogen in acid chloride solutions. The value of this index for various alloys has ranged from about a low of 26 for type 316L stainless steel to a high of over 50 for the most advanced nickel-base alloys, whose costs run four to seven times that of 316L.
One of the earliest broad spectrum alloys of outstanding resistance to seawater and other chloride solutions was a nickel-base alloy marketed under the tradename, Hastelloy C, which nominally contained, by weight, 16% Cr, 16% Mo, 4% W, 5% Fe, a few impurities and the remainder Ni. This alloy was furnished in both cast and wrought forms and had to be given a solution heat treatment at about 2100.degree. F. prior to use in service. This alloy has undergone several modifications over the years due to the problems with corrosion in weld areas because of the precipitation of Mo-rich and W-rich intermetallic phases. Also, while the alloys of this type have good resistance to reducing acids they have limited resistance to oxidizing acid environments. One version, known as Hastelloy C-22, designed to provide improved resistance to oxidizing acid environments, contains nominally 22% Cr, 13% Mo, 2.5% W and 3% Fe with the balance essentially Ni.
The nickel-base alloy, Inconel 625, contains about 22% Cr, 9% Mo, 4% Cb plus Ta and 61% Ni and has somewhat better resistance to strong oxidizing substances than the Hastelloy C family of alloys; however, it is still a premium, high-cost material. Also it is now known that, columbium (niobium) is somewhat detrimental to resistance to chlorides, so that Inconel 625 is not as resistant in the presence of strong oxidizers as might be expected from the Cr and Mo contents of the alloy.
Binder, U.S. Pat. No. 2,777,766, discloses alloys of 18% to 23% Cr, 35% to 50% Ni, 2% to 12% Mo, 0.1% to 5% Cb plus Ta, up to 0.25% C, up to 2.5% Cu, up to 5% W, and the balance iron and impurities. While none of the exemplary alloys employed any tungsten, the commercial alloy derived from the patent and sold under the tradename, Hastelloy G, nominally contained about 6.5% Mo and up to about 1% W. This alloy has rather poor resistance to chlorides relative to its alloying elements content, probably due in part to its high columbium content.
Hastelloy G was modified to increase its molybdenum content to about 7.1%, and reduce columbium and tantalum content to about 0.5% while keeping the tungsten at about 1% or slightly less. This modified alloy, Hastelloy G3, still suffered local corrosion in U.S. Navy tests in filtered seawater.
Henthorne, et al, U.S. Pat. No. 4,201,575, discloses an alloy marketed under the tradename, 20Mo6, which nominally contained about 35% Ni, 25% Cr, 6% Mo, 2% Cu and the balance essentially Fe. This alloy was intended for service in acid chlorides, and at about 30% Fe content, had departed considerably from the Ni-base category. However, it was prone to intergranular attack after welding unless it was subsequently solution heat treated at high temperature. Furthermore, it did not meet expectations for broad spectrum corrosion resistance, possibly due to its marked tendency to form microstructural transformation phases.
DeBold, et al, U.S. Pat. No. 4,487,744, discloses an alloy known as 20Mo4 which nominally contains 37% Ni, 23% Cr, 4% Mo, 1% Cu, 0.2% Cb and the balance essentially Fe. This alloy was intended to avoid the problems of alloy 20Mo6 and still have resistance to chlorides and a broad spectrum of other corrosive solutions. It has fairly good resistance to sulfuric and nitric acids and is not susceptible to intergranular attack after welding, but it is not completely immune to local corrosion in seawater.
Liljas, et al, U.S. Pat. No. 4,078,920, discloses highly modified 316L stainless steel and contains nominally 18% Ni, 20% Cr, 6.2% Mo, 0.8% Cu, 0.2% N and the balance essentially Fe. The alloying approach of this alloy provides excellent resistance to local corrosion in seawater and to many reducing substances but only moderate resistance to oxidizing solutions.
High-manganese austenitic stainless steels of reduced nickel contents have sometimes contained nitrogen and molybdenum, but none of these steels offer very high resistance to acid chlorides. A high-silicon version of these steels was said to have increased chloride resistance, but neither that version nor any other high-Si alloy so far developed has been able to provide significantly increased resistance to acid chlorides.
Economy, U.S. Pat. No. 3,565,611, discloses alloys for resistance to stress corrosion cracking in caustic alkalies which are composed of 18% to 35% Cr, up to about 7% Fe, and optional amounts of up to about 3% each of V,W,Ta, up to 1% Cb, up to 4% Al, up to 1% Ti, up to 6% each of Cu, Co and Mn, and the balance essentially Ni. The claims require that the sum of V, W, and Mo is less than 6%. While molybdenum and tungsten are both optional in the alloys of Economy, it is well recognized in the field that there is no known austenitic Ni-base or Fe-Ni-Cr base alloy that contains no molybdenum and still has resistance to chlorides.
Goda, et al, U.S. Pat. No. 3,811,875, discloses austenitic stainless steel alloys containing 10% to 26% Cr, 4% to 46% Ni, 0.5% to 10% Cu, 0.25% to 2% Al and optionally up to about 3.5% Mo. Goda further claims that up to about 7% W may optionally replace all or part of the molybdenum. However, as stated above, such alloys, devoid of molybdenum, do not have significant resistance to chlorides.
Kudo, et al, U.S. Pat. No. 4,400,349, claims alloys resistant to chlorides containing 20% to 60% Ni, 15% to 35% Cr, 3% to 20% Mn, up to 24% W, 0 to 12% Mo, up to 2% Cu, optional amounts of Co, Y, Ti, Mg Ca and rare earths, with the balance essentially iron and impurities. Kudo further specifies that the weight percent of chromium plus 10 times molybednum plus 5 times tungsten taken together must exceed 50%. The exemplary alloys of Kudo encompass more restricted portions of the disclosed compositional ranges, but still provide only tensile elongation values of 8.7% to a maximum of 26%, with the vast majority of the alloys falling below about 20%.
Asphahani, et al, U.S. Pat. No. 4,410,489, discloses alloys said to be particularly resistant to phosphoric acid and composed of 26% to 35% Cr, 3% to 6% Mo, 1% to 4% W, 0.3% to 2% Cb plus Ta, 1% to 3% Cu, up to 1.5% Mn, up to 15% Si, 10 to 18% Fe, and the balance essentially Ni. An alloy offered under the tradename, G-30, nominally contains 43% Ni, 30% Cr, 5.5% Mo, 2.5% W, 2% Cu, 1% Si, 0.8% Cb and 0.03% C. This alloy is claimed to provide excellent resistance to a variety of severe environments, especially hot phosphoric acid.
However, alloy G-30 contains such large amounts of chromium along with the other strong ferritizing elements, molybdenum and tungsten, that it tends to form sigma and other detrimental phases when slowly cooled from the molten state, as in the production of large castings. Also, despite its low carbon content, alloy G-30 tends to suffer attack by many corrosive substances in the weld and heat affected zone, and accordingly large castings or welded castings are solution heat treated at high temperatures before being put into service.
In addition, duplex stainless steels have developed rapidly and have received wide acceptance in many types of chloride service. The newer duplex stainless steels have adequately answered the welding problems for wrought product assemblies, such as pipe lines, but both cast and wrought duplex stainless steels of superior resistance to chlorides must be solution heat treated at some stage of production.
Therefore, there have been no prior art alloys of any type that have essentially complete resistance to seawater and generally excellent resistance to more severe chloride solutions as well as a wide range of other solutions, that do not require solution heat treatments at high temperature. Such heat treatments applied to large castings are costly, difficult and severe, in that they lock in high stresses in the final shapes and often cause cracking or distortion.
Thus there has remained a need for alloys of moderate cost, excellent resistance to chlorides and to other substances, high tensile elongation and weldability, but which do not require solution heat treatments prior to service, even after welding.